1. Field of the Invention:
This invention relates to a focusing screen and more particularly to a focusing screen having a plurality of split prisms in the central portion thereof.
2. Description of the Prior Art:
Heretofore, there have been known focusing screens of the type in which a split prism arrangement is provided in the central portion thereof. The central portion is divided into an upper area and a lower area, and on each area a split prism is formed with the slant directions of the upper and lower split prisms being different from each other. One prism is slanted rightwardly and downwardly and the other is slanted leftwardly and downwardly. In such a focusing screen, an image of a rod-shaped object extending in upward and downward directions can be deviated to the left and right to show an out-of-focus condition thereof. However, with such a conventional focusing screen, it has been impossible to detect the condition of focusing on a rod-shaped object extending in the left and right directions.
To solve this problem, U.S. Pat. No. 3,602,085 discloses a focusing screen wherein the central portion of the focusing screen is divided into four areas with vertical and horizontal lines which intersect at the center of the screen; and on each of the divided areas a split prism is arranged, each split prism having a different slanting direction so that the focusing condition can be detected on rod-shaped objects extending horizontally as well as objects extending vertically.